For certain applications, there is a need for a broadband, high power communication system. For example, in military applications a broad bandwidth is required for secure spread spectrum communication and high power is required for long range. High power broadband communication systems require high power broadband antennas. Often these antennas have an input impedance that does not match the desired transmitter or receiver with which it is used. In such circumstances, impedance transformers can be used to transform the impedance of the antenna to the impedance of the transmitter or receiver.
Transmission-line transformers may be used for matching networks for antennas and amplifiers operating at radio frequencies, such as frequencies in the HF and VHF bands. They are also known for having low losses, which makes them especially useful in high power circuits. Transmission-line transformers may be made using various structures, such as parallel wires, coaxial cables, or twisted wire pairs. In the case of using coaxial-cable transmission lines having the correct characteristic impedance for the transmission-line transformer, the theoretical high frequency bandwidth limit is reached when the cable length comes into the order of a half wavelength (λ/2). By introducing magnetic materials such as powdered iron or ferrite to the transmission-line transformer, both the low frequency limit and the high frequency limit may be improved.